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What is the result of increasing number of propeller blades on the same engine
For example Rotax 912. We can meet this engine with 2 and 3 propellers mounted. Or compare airplanes equipped with engines with pretty the same power output i.e. C152 with Lycoming O-235 - 110 HP and 2-blade propeller versus AT3 with Rotax 912 - 100 HP and 3-blade propeller (also both engines have similar max propeller RPM (Rotax has reduction gearbox)). I know that increasing number of propeller blades alllow better power absorption, but what does it change if the engine has pretty low max power?

I don't think so, because the engine power is the same or almost the same, but the number of propeller blades is changed. So what why it is changed, because no extra power is "added" to absorbed by more propeller blades.

This might very well be for noise reasons. Lowering the lift coefficient on the blades reduces noise a bit. The three-bladed propeller is slightly less efficient, but produces less noise. One or two decibels might mean the difference between being allowed to fly and being grounded in some airfields, especially in Europe.

What it means in practical terms is if you replaced a 2 blade Warp Drive prop on a 100 HP engine with a 4 blade prop with double the total blade area, and if the blades of the 4 blade were set to the same pitch as the 2 blade, the torque requirement of the 4 blade would be significantly higher and you'd find the engine can't make the same static RPM it was making with the 2 blade at wide open throttle.
You would have to re-pitch the blades of the 4 blade prop to a lesser angle and retry until you regained the original static RPM.  You're basically offloading the individual blades until the total torque demand matches the original prop.  You might find you've lost a lot of efficiency from the blades now operating at less than their optimum blade angle of attack.
If the 4 blade prop had much smaller individual blades, so the total blade area was the same as the 2 blade, the difference in pitch would be minimal, although some adjustments in pitch would still need to be made to match static RPMs, and the static thrust you are getting at the target RPM would be reduced somewhat from the 2 blade because of the lower efficiency of the 4 blade.
If it was a constant speed propeller, the propeller governor would be doing all that for you.
If you are after the maximum thrust efficiency for a given horsepower, 2 blades are always better if the power can be absorbed and the target RPM can be achieved with 2 blades.  What makes multiple blades attractive, as a conversion to an existing engine, is the desire to limit noise, or limitations on diameter due to tip speed limits or ground clearance requirements. For example, 3 blade propellers are popular on DeHavilland Beaver seaplanes because the noise levels are lower because the tips speeds are lower, and the shorter blades get a little bit less water spray erosion damage, benefits sufficient to live with a small reduction in prop efficiency compared to the 2 blade paddle prop it originally had.


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Is gascolator and fuel strainer the same thing?
I am wondering is gascolator the same as fuel strainer, or this two things are bit different. Because sometimes I see one term, sometimes another, and I don't know if I am reading about the same thing.

Yes in practice they are the same thing (it's surprisingly difficult to find the origin of the term "gascolator") and most definitions will mention both terms, although I tend to think of the gascolator part as the gravity water separator bowl/quick drain component, and the strainer as the filter screen covering the outlet.
Back when cars had them (up to the early 60s) they were called "filter bowls" with the bowl part made of glass and without the quick drain.  You just removed the bowl to get rid of any water you saw when checking under the hood (airplane gascolators from the 40s also used glass bowls, although most have been replaced by metal ones).
Most aircraft gascolators use a 75 micron mesh screen (the strainer part - about like the screen on those plunger coffee makers) at the top of the housing to keep particles from passing to the carb.  On my homebuilt I improved on that by using a 30 micron sintered bronze filter made for the filter bowl of a late 50s Chev Impala, which fits perfectly in the standard gascolator bowl.

So gascolator is a fuel strainer equipped with a bowl, where water and debrees found in fuel are collected and can be drained by a pilot to check the fuel quality?

how debrees go to the bottom of the bowl? Shouldn't the filter bea blockage for them to pass?

Yes.  It's a combination water separator and filter screen.   Fuel enters at the center in the roof of the housing, and exits at another point in the roof (the screen covers the exit), so it has to move down into the chamber made by the bowl, then back up while the velocity of the flow slows way down because it's moving into a large chamber.  Water and solid particles (mostly) settle to the bottom and whatever particles remain in suspension are caught by the screen.  You get in trouble when enough water enters the bowl to fill it and now it moves into the carburetor and oops.

Short answer: In practice they are often considered the same device, however they are really different devices.
The gascolator is used to separate different densities.  Water is the normal objective, however tank sediment (sand, particles of sealant and other debris) are secondary objectives.  The gascolator, of necessity, should be at the lowest point in the fuel system.
A fuel strainer is normally a screen or other mechanical filter, which separates by virtue of particle size.
Often the design of these devices are done so that a strainer screen is flushed by fuel sampled from the gascolator, preventing obstructive build up of debris.
In many small single engine and light twin engines, the thing often called the gascolator is just that, and has minimal straining function.  A risk is straining is that the strainer could get clogged with contaminated fuel.  Such a clog can prevent fuel flow, and cause fuel starvation of the engine.  Generally it is desirable for an aviation engine to accept fuel with some particulates rather than experience fuel starvation due to a clogged strainer.
So in the end, what has traditionally been called a gascolator is implemented, and an absolute particle size strainer is not as common or strains for a relatively large size.

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What are some of the advantages of inverse-tapered wings?
Inverse tapered wings are rarely used in aviation. The only legitimate plane I know about with inverse-tapered wings is the XF-91 Thunderceptor. So why aren't they used. What are the disadvantages of these wings? What are the advantages? Why aren't these wings used more in planes?

Hello! I edited the part about stall characteristics out because it's better to avoid asking two questions in one post. That is a good question though; maybe you could post it separately?

Some of the arguments that hold for other 'exotic' shapes hold for the inverse-tapered too:
Why was the Spitfire's elliptical wing almost uncopied by other aircraft of World War 2?

Two words: Structural nightmare.

@Pilothead Yes this came to mind when I posted this

As I understand it, it was originally designed to solve the problem of wingtips stall at low speed, due to stagnant air, moving outboard along the wing, and altering the angle of attack near the tips. The downside, of course is that larger and heavier wing tips increase the moment of inertia about the longitudinal axis And most likely give the airplane unfavorable stall and spin characteristics not to mention problems with flutter. A lot of the wingtip stall problems were eliminated with improvements to wing design geometry, as well as the inclusion of wing fences to control movement of airflow around the wing.
Basically, an interesting solution to a problem, but it’s not the best answer and the negative aspects outweigh the benefits to the design.
I will wait to see what Peter Kämpf’s opinion is of this..

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Help identifying my great grandfather’s biplane
I was wondering if anyone could identify the plane my great grandfather (Francis W. Reed) is standing in front of? The picture was probably taken in Indiana.

Can you tell us where and when this photo was taken?  Are there other photos of the same airplane?  (Can you also state your grandfather's name and any connection to the airplane - pilot? mechanic? pilot's buddy?)

Unfortunately I do not have a location or date for the photo (most likely Indiana). My great grandfather lived in southeastern Indiana and from what I was told he owned and piloted this airplane. His name was Francis W. Reed. Thanks for you help!

A timeframe might also be helpful

My first thought was a Tiger Moth (OY-DIH, perhaps) but the bracing between the wings doesn't match. Shape of the fin does though. These old biplanes were all extensively modified, so it's always hard to be 100% sure.

My great grandfather is on the left with the bandana. I believe he is in his 40s and was born in 1901. Thank you all again for the help!

I am not sure it’s a Tiger Moth. The rear horizontal stabilizer sits below the the fuselage. From all the photos I have seen, the Tiger Moth’s sits flush with the upper ridge of the fuselage.

@Tman I agree, but my gut feel is still some sort of De Havilland

The horizontal stabilizer position, as Than mentioned, doesn't seem to match any Moth biplane. https://en.m.wikipedia.org/wiki/De_Havilland_Moth It's getting late here, but I'll do more research tomorrow. (cc: @Jamiec)

This aircraft is an Alexander Eaglerock A-4 registration NC701H built in 1929 by the Alexander Aircraft Company
There's not much information about this exact aircraft, but the same type features in a great article about The First Black Airmen to Fly Across America. It turns out there is at least one A-4 still flying - NC6927 - which you can see being flown in a short video about one of those aviators, James Banning.

 source

Eaglerock A-4 (source) - note the deep sides for the forward cockpit, which you can see in your picture.

Eaglerock A-3 (source) - although a different engine, this angle helps makes sense of the struts in your picture.

Oh good find!! I think this may well be the one.

At last! Good job

That is a big airplane-- the guys in the original photo must have been standing quite some ways in front of the fuselage, to make the plane look smaller.  (Wingspan is large, and they appear to be standing near wingtip.)  PS -- second photo above-- looks like a rather low seating position-- how the heck could the pilot see anything at all during the takeoff run?

The pilot sat behind. The forward cockpit could seat two passengers side-by-side. You are right about the size, I believe the first picture is a montage, I will replace.

Yes I realize pilot sat behind, I was looking specifically at the last of your pictures when commenting on visibility--

Wow, that’s it! Thank you so much everyone for the help! Fascinating history as well.


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In case of a drone strike with an airplane below 400 feet, who is at fault?
If a plane is flying VFR (see and avoid) below 400 ft, and the pilot collides with a drone due to not seeing the drone until it's too late to take evasive action, then would the pilot be at-fault for insurance purposes? Would the drone pilot (or the airplane pilot) have violated any federal regulations for not keeping clear of the other craft?
Conditions:-

Class E or G airspace
out of the 5nm radius of an airport
sparsely populated area
operator of drone is not within 500 ft of where the collision occurred (i.e. for the purposes of FAR 91.119 (c)).
I have a smaller piston engine plane in mind, but I'm assuming that the outcome would be the same for faster jet aircraft too

What is the airplane pilot doing flying below 400 ft. more than five miles from an airport??

@Lnafziger: I suppose "crop dusting" could be a possibility.

@Lnafziger -- cropdusting, pipeline/wireline patrol, landing out

Question arises, what is an airplane doing at 400 feet anyways? Isn't it illegal to get that close to the ground in most places?

Common reasons are crop dusting, fire-fighting (i.e. application of fire retardant), and utility patrol. Recreational GA planes might go low for sight-seeing reasons, although preparation (surveying for obstacles and emergency landing sites) and extra awareness would be necessary.

@Alexus It's not illegal over water or over "sparsely populated areas." Perhaps unwise unless you have a really good reason (like crop dusting,) but not necessarily illegal.

The person at fault is whoever has the cheaper lawyer.

@Alexus How about off airport (bushplane) landings, or even just simulated "engine failure" emergency "landing" practice? During my PPL training I've had a CFI definitely get me below the 400 foot level to demonstrate the energy management to end up in a position to land right after the fence in an unoccupied farmers field instead of right before it...

The FAA has some rules pending about this, you can read them here but for drones under 55 LBS one of the proposed rules is,

Must yield right-of-way to other aircraft, manned or unmanned.

and since you bring up class E it should be noted that

Operations in Class B, C, D and E airspace are
  allowed with the required ATC permission.

if/when this goes through it looks like its the drone operators fault since the plane had the right of way
According to the FAA's website in relation to model aircraft they state that,

(4) the aircraft is operated in a manner that does not interfere with and gives way to any manned aircraft;

this is pretty much a blanket statement that maned aircraft have the right of way but the FAA handles most things on a case by cases basis so in the end of the day it could go either way.

Drone strike with plane below 400 feet - who's at fault?
According to this article (FAA to Drone Operators: Keep Off Airports, Stay Below 400 Feet) it appears that the pilot of the plane is guilty for the collision. If the drone operator respected the FAA rules (line of sight, below 400 ft and max. allowed weight, far from the airports) he can not be declared guilty for the accident.
Federal Aviation Administration (FAA) chief Michael Huerta says the agency already has regulations in place to ensure safety in the skies.
Huerta says the agency already has rules that prohibit operators of personal drones from flying into airports and soaring over 400 feet in the air., Source

Thanks -- I'm looking for references to FARs and/or other codified rules.

Your comment that the drone operator "cannot be declared guilty" is highly doubtful; it's well established for manned aircraft that even if you operate in compliance with all FAA regulations, you can still be cited for "careless operation" under 14 CFR 91.13, i.e. legal is not the same as safe, and operating unsafely is illegal. What happens if you decide to use your drone to film a crop duster in operation at 50ft and you get in his way? Still not your fault? Until the FAA gets around to publishing their drone rules, any situation like this is unclear.

Just because you're operating in airspace where you have the right to be doesn't mean that you "can't be declared guilty" for a mid-air collision. The airplane has the right to be there, too. The drone is much more maneuverable and the drone operator is much more able to see (and hear) the approaching aircraft than the other way around. If the drone operator is complying with regulations that he be within visual line-of-sight of his, then he will be able to see and hear the aircraft and failure to get out of its way would definitely be negligent (and probably grossly negligent.)

Nothing in the article you link supports your claim that the pilot of a plane is always guilty. The article only says that it's prohibited to fly drones higher than 400ft and out of line of sight. That says nothing whatsoever about who is responsible for collisions between drones and planes below 400ft.

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What are advantages and disadvantages of Rotax and Lycoming engine designs?
I am wondering what are advantages and disadvantages of these two designs, e.g. Rotax 912 vs Lycoming O-235, compared to each other. At first glance they are pretty similar (both are 4-cylinder, horizontally opposed, with carburetors, naturally aspirated, with similar horsepower output etc.) but there are also some differences (engine displacement, maximum RPM, preferred fuel, oil system and consumption etc.) Can someone explain pros and cons of both designs?

The biggest difference between these two engine designs is that the rotax contains a reduction gearbox which allows it to turn faster and hence develop more power per cubic inch of displacement, and the lycoming is a direct drive engine limited to a rotating speed that maintains the  prop tips subsonic. This gives the rotax better power-to-weight ratio  but causes it to run hotter and shortens its time-between-overhauls. In addition, the gearbox itself requires overhauling which adds to the overhaul costs. In practical terms, air-cooled & geared engines for GA aircraft have not proven as popular as direct-drive engines in similar applications; in fact it is common to replace geared engines with direct drive engines with larger displacement to improve reliability and reduce rebuild costs.

Another difference stemming from the higher heat produced by the Rotax is that it has to be liquid cooled (partially) like an automotive engine.

Hmmm. I've seen Lycoming/Continental to Rotax conversion kits but not the other way around, what have I missed?

Thank you frr your answer! Do you know what are pros and cons of using small cylinders and high max rpm (Rotax) vs big cylinders and low max rpm (Lycoming)?

small cylinders and high RPM mean the engine weighs less but wears out faster and is subject to catastrophic failures due to overheating/cooling system issues. big cylinders and low RPM mean the engine is heavier but lasts longer, and less subject to overheating/burnt valves.

And no change in power output / fuel consumption / efficiency?

you would need to check the engine data sheets for that information. -NN


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How does an extended slat generate drag?
I am wondering how does an extended slat generate drag because I often read that slots agument lift by increasing critical angle of attack, but they do generate some extra drag. I am curious what is the cause of this drag.

Do you mean slat?

Friction drag is proportional to the speed gradient right at the wall. This gradient is highest when the boundary layer is thin, and every boundary layer starts at zero thickness, with thickness increasing downstream.
Slots on purpose allow a fresh boundary layer to form on the wing or flap downstream of them because this fresh boundary layer can overcome steeper pressure rises downstream.
The downside of this fresh boundary layer is a lot more friction drag compared to the retracted state where the boundary layer of the slat becomes the (now much thicker) boundary layer of the wing. The same is happening again with the flaps: Slotted flaps allow the highest lift increases, but at the cost of much increased drag. However, would there be no new boundary layer on the flap, the flow would separate much earlier, at much lower flap angles and angles of attack.
Another effect are higher speeds of the flow around the wing at high lift coefficients. Low pressure means high speed, so the flow speed outside of the boundary layer is higher when local pressure is lower. This increases the speed gradient in the boundary layer, thus adding another drag increment,

The slat projects downward increasing frontal area, and   forms a convergent duct leading from below to above the LE, that forces air to go around sharp corners to move up through the slot.  Plenty of turbulence, extra frontal area, and air forced to make sharp changes in direction.  Of course it's going to be draggier than a clean leading edge.
The effect is so strong that on the CRJs, the landing technique is completely different from the 200, with no slats, to the slightly larger 700, with slats.  The 200 just coasts when you pull off the thrust, and you go to idle at 50 ft.
The 700, you have to keep the power on into the flare and ease it off at 10-20 ft, or else you will come down like a brick as the speed rapidly bleeds off. Using 200 technique on a 700 will guarantee a hard landing.

All passive High Lift devices (e.g. nothing that uses active flow control) increase the Lift Coefficient by increasing the overall camber of the wing system (the wing + slats + flaps).
If the wing stays at the same angle of attack, and the slats + flaps are extended, the increased camber is going to increase the lift coefficient. The benefit of this is that the aircraft can now fly slower and produce the same amount of lift to maintain level flight. The detriment of this configuration is the increase in induced drag.
As we know from the induced drag relationship, the induced drag coefficient scales with the square of the lift coefficient, and so the extra lift generated by slats + flaps comes at a significant expense of drag.
Vorticity is shed at any point that we have a spanwise change in circulation, including at the edge of slats & flaps. Therefore even modern wingtip winglets do not minimize the induced drag penalty that comes from these high lift devices.


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Why does the condensation cone disappears when the airplane starts flying supersonic
In this answer How are condensation cones created by supersonic airplanes? it is said that the condensation cones appear only when the airplane flies at speeds below Mach 1. I am wandering why does it disappear above Mach 1, and does the condensation cone appear when flying exactly at Mach 1?

Exactly Mach 1 isn’t really a thing, as the airflow will accelerate to some degree as it passes over each aerodynamic surface.

Condensation occurs when the air accelerates, which is accompanied by drop in pressure and temperature that can push it below the dew point if humidity is high.
As explained in What is compressibility drag?, at subsonic speeds the air accelerates and expands to make way for the aircraft, but it compresses and slows down at supersonic speeds. So the condensation only occurs at subsonic speeds.
Now the condensation can be seen under various circumstances when the humidity is high: over the wings of landing planes, over the wing roots of planes pulling high G, in the core of the wake vortices, and also inside a lambda shock at transsonic speeds, which is the condensation cone.
The transsonic condensation cone is particularly noticeable because, as explained in What is compressibility drag?, near the speed of sound the decrease in pressure leads to decrease in density, so the air must accelerate to a much bigger distance from the aircraft to make a room for it, making the condensation cone much bigger than the other cases of condensation around the aircraft.


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Why does a low pitch propeller have better acceleration/take-off and climb characteristics than a high pitch propeller?
Why does a propeller with a low pitch have better acceleration/take-off and climb characteristics vs. a high pitch propeller?
And conversely, why is a propeller with a high pitch not as good during a climb, but is good for a cruise phase?
I am looking for a „physics and maths” explanation, I like the bicycle and car gears analogy and they are really intuitive, but they do not explain why does it works like that.

One way to look at it if you ride a bicycle: how hard is it to get started in high gear, or to climb a hill in high gear? How is cruising on a flat stretch in a low gear vs. a high gear? It is very similar for propeller pitch in airplanes. A single speed bike is much like an airplane with a fixed-pitch propeller -- a compromise.

So, in reality, you want the prop to take the same "bite" of air (at its optimal AOA), based on relative wind created by forward airspeed (higher in cruise) and rpm (lower in cruise).

Low pitch advantage
Power
A lower pitch allows the blade to spin faster ($\omega$) for the same torque ($T$). The relationship between power ($P$) and speed and torque is $P = T \omega$, so this means that for the same $T$ we get more power out of the prop when it spins at a faster $\omega$. More power means a higher climb rate.
Stall
At low airspeed, significant sections of a high-pitch propeller will be stalled. This translates into lost thrust, and explains why a plane with a climb prop has a shorter ground roll than the equivalent with a cruise prop.
High pitch advantage
Lower tip speed
Efficiency goes down at high tip speeds, and basically falls off a cliff when the tip speeds reach the speed of sound. So a higher pitch allows for creating the same thrust at a lower tip speed, and thus gives higher efficiency.
Engine RPM
A secondary effect is that a prop which is spinning more slowly typically leads to a more efficient engine, and reduces wear. So while this isn't directly related to your question about the prop being more efficient, it is a very important effect in the overall system efficiency.
Additional reading
Do propeller coefficients go to zero at the same value of advance ratio? gives a good explanation of propeller behavior for different pitch.

And by a torque you mean engine output, right? Because on some propeller forces schemes there is a force, perpendicular to axis of rotation, also called torque, what is weird to me, because I always thought that torque = force x arm

That's correct, I mean engine torque. This second torque you're referring to could be gyroscopic precession or p-factor, both of which have important impacts on the plane's flight dynamics.

And, if I'm understanding this properly, with higher the pitch, power and thrust output is lower, but the torque remains the same?

And is engine torque constant as we change airspeed/rpm/propeller pitch?

Engine torque is constant-ish. https://wildnordics.com/wp-content/uploads/2020/06/Screen-Shot-2020-06-27-at-12.28.09-pm.png gives torque for a Rotax. If you look at the y axis, you can see that it varies +-10% across the 3000-6000RPM range. At any throttle setting, the prop will speed up until the load torque equals the output torque. A higher pitch prop has a higher load torque for the same RPM, which conversely means that for the same engine torque it won't spin as quickly.

What is a load torque? Pardon for my ignorance

A load is a general term for the thing which is being driven. So in your house, the refrigerator and lightbulbs are "loads" on the electric grid. In a bus, the "load" is the wheels. In a plane, the "load" is the propeller. The word "torque" serves to differentiate the load from other load types, for instance a voltage load or a pressure load.

One more question, why it is „harder” for an engine to rotate a propeller with a higher pitch?

@niels nielsen gives a lay description in his answer, which might be what you're after. If you want a little more formal of an answer, a higher pitch has a higher angle of attack. A higher AoA produces more lift. More lift produces more induced drag, and more induced drag makes it require more torque for the same RPM.

Thank you for all your answers, all is clear now

Do you know why moving the propeller control forward increase drag?

This might be enough out of scope that it should be its own question. Think you could open a new one? You might even find it's already been answered.

I was searching for it and I didn’t find it :( I’ll open a new one.

A low-pitch or "climb" propeller takes smaller "bites" of the air and allows the engine to run all the way up to its maximum speed, and therefore produce its maximum rated power setting (don't make the pitch too fine, or the engine will overspeed!). The fuel burn rate will then be at a maximum and you'll get maximum acceleration and climb rate.
A high-pitch or "cruise" propeller takes bigger bites and therefore loads the engine down to the point where it is turning slower than its redline limit and developing less power at wide-open throttle. This lower power setting lets you obtain the most economical cruise in level flight by reducing the fuel burn rate- but it prevents the engine from running at its maximum power setting during a climb. Therefore a cruise prop gives you a lower acceleration rate, and lower rate of climb.


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Is baro-VNAV the only possible source of vertical guidance on LNAV/VNAV approach?
As written before, is baro-VNAV the only possible source of vertical guidance on LNAV/VNAV approach? I read somewhere that SBAS is also a possible source, but in that case wouldn’t it be a LPV approach then? As far as I know when you load a RNP approach into your avionics you don’t have a choice which one you fly (e.g. LPV or LNAV/VNAV - assuming we are certified for both).
I would be grateful for any official acts references.
To clear it out - how it is possible to shoot LNAV/VNAV approach with SBAS augmentation, because I thought that every SBAS augmented RNP approach must be LPV.

Sadly I don't remeber where :(

I believe @busdriver may have overlooked something. Your TSO 145 or 146 avionics will "fail down" from LPV (a more constrained lateral and lower minima approach) to LNAV/VNAV (wider horizontal guidance through final - higher vertical minima) if WAAS accuracy is degraded, so yes, you can fly LNAV/VNAV to a DA with an SBAS GPS even if not baro-aided.
Your box will always choose the "best" approach available - i.e., the one with the lowest minima. If there's an LPV approach available and the signal is good, you'll be offered that by your box. If there's only an LNAV/VNAV approach for the airport or your box or the WAAS signal is degraded, you'll be served up an LNAV/VNAV, or if the signal/box has major problems, just an LNAV approach.
In the AIM 1-2-2 b(1)(a)(1) it states:

LNAV/VNAV incorporates LNAV lateral with vertical path guidance for systems and operators capable of either barometric or SBAS vertical.

See boldmethod.com for a great comparison of the RNAV GPS approaches.
As GPS is more reliable (if the system is up...) than temperature sensitive baro-aiding, that does make sense.
What I'm not clear on is if the LNAV/VNAV step down fix altitudes need to be honored even if flying the glideslope with WAAS. One's avionics manual may dictate that - I haven't dug through mine to find out. It seems like an overabundance of caution and a relic of when these were flown only baro-aided, but something I'd be willing to do for safety sake. If anyone has a quotable source for an FAA opinion on that, I'd be grateful.

Yes. While APV Baro-VNAV and APV SBAS share similar lateral design criteria, the vertical obstacle clearance surfaces are different. In APV Baro-VNAV procedure design is based on barometric reference as vertical guidance so that is to be followed. If LPV minima is published, also APV SBAS is then considered. In practice, the approach plate is the same with two (or three with LNAV) set of minimums.
At least in Airbus 350, the only equipment I've flown APV SBAS approaches with, there is an option to deselect the "Satellite Landing System" (or SLS) as Airbus calls it. That will leave you with "FMS Landing System" (FLS) which has barometric vertical guidance.

Thank you, I understand that, but one question - the "true" LNAV/VNAV approach (with linear guidance all the time, etc.) can only be flown with baro-VNAV yes? If we chose SBAS as our augmentation we will then fly LPV, not LNAV/VNAV right?

That is correct


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What is the role of a bound vortex?
From what I know the term bound vortex comes from potential flow theory and is a part of circulation around a wing.

Does it mean the same as circulation or lift?
Is it a cause of generating lift over a wing? If yes or partially yes, what is its role in that?

I'll use the following diagram for illustration:

Intuitively, a hard surface resists fluid flow via shear stresses; these shear stresses tend to rotate fluid elements (i.e. vorticity). Therefore, we can regard a hard surface as filled with vortex sources.
In actual fact, only the boundary layer contains vorticity, and there is usually no fluid element rotation outside of it. Since the boundary layer does not affect lift [much] in attached flows at high Reynolds number, we can ignore it and call the entire flow irrotational. This leads to the concept of potential flow. Furthermore, we can model the hard surface as a vortex sheet. The vortex sheet is called singularities and does not affect the irrotational assumption because it is a boundary and not part of the flow itself.
We can simplify the picture further. If the body is thin (like a wing), we can model the entire wing as a single vortex sheet (collapsing both upper and bottom surfaces into one). To simplify further, we can break the vortex sheet into individual vortex filaments running parallel to the wing (in the y-direction in the picture). These vortex filaments are called bound vortices. For high aspect ratio wings, we can simplify even further by considering the entire wing as a single filament, which leads to the Lifting Line Theory.
Since a vortex filament cannot end in a fluid, it must shed and end in the far field at infinity. This is a horseshoe vortex; the shed vortex filaments are called trailing vortices. A filament does not only shed at wingtips; in fact, it's shed everywhere along its length.

Image ref: https://history.nasa.gov/SP-367/f53.htm
Circulation is the total vorticity within a given closed loop. If you draw a closed loop from leading edge to trailing edge at a given span location, the total vorticity passing through this loop is the circulation at the given span location. Note that trailing vortices would be parallel to the said loop and will not partake in the circulation.
Lastly to complete the picture, acccording to Kutta-Joukowski Theorem, lift at unit span ($L'$) is directly proportional to the circulation ($\Gamma$) at the location:
$$L'=\rho_\infty V_\infty \Gamma$$

Okey, thank you! But on your picture you replaced lift vector by bound vortices, and the lift equation from KJ theorem that you pasted include circulation. Is this circulation just bound bortex or whole circulation - bound vortex and tip vortices? And is bound vortex the cause of lift, no bound vortex, no lift?

And is bound vortex equivalent tu flow over and under the wing if we ommit vortices that leave the wing (tip vortices and starting vortex)?

@Konrad Second last paragraph: "Note that trailing vortices would be parallel to the said loop and will not partake in the circulation."

@Konrad Re your sec question: it depends on the fidelity of your modeling as mentioned in the post. If it's a lifting line, then the flow field from the circulation would only be very approximate, but good enough for lift estimation. If it's lifting sheet, then it gets better. If it's lifting surface, then even better.

Thank you, one last question. Is the bound vortex a cause of lift generation? Because if we assume it as an airflow around a wing, so logickly no airflow around the wing means no lift

@Konrad Circulation is the cause of lift generation. Bound vortex, or systems of bound vortices are models used to generate the corresponding circulation.


text:
A plane is flying at constant velocity in equilibrium, then pitches up. What happens?
This might be a dumb question. And I could probably work it out myself. But here goes! Assume a plane is flying along at some velocity $v_0$ at angle-of-attack $a_0$, using throttle setting $tr_0$. Forces are balanced. At time $t=0$ the pilot gives input to the elevators to pitch the aircraft up, changing nothing else. What happens to the aircraft as a result (in terms of its position and velocity)?
Note that this differs from the traditional "climb" free-body diagram because we are not necessarily in equilibrium.
In terms of just instantaneous forces the second after the pitch changes:

The angle of attack is now increased. Therefore more lift is generated.
However, now some weight is added to drag component, and since the pilot has not increased the thrust from the engine, the plane experiences a net force downwards and backwards.

But not sure how to add all these up to get a time course of position and velocities.

Does the elevator remain fixed or go back to an equilibrated state? Nothing else is touched?

It's helpful to keep in mind-- any net force acting perpendicular to instantaneous direction of flight path will curve ("bend") the flight path up or down.  And any net force acting parallel to instantaneous direction of flight path will cause airspeed to increase or decrease.  With this in mind, it should not be hard to go through each step in the process.  Obviously, any upward or downward bend in the flight path changes the magnitude of the tangential and perpendicular components of the weight vector...

What was your own initial Answer, and why did you not trust that?

A step increase in elevator (which is then held) will first upset the pitching moment equilibrium of the aircraft -- the pitching moment will be non-zero.  It will pitch nose up, increasing angle of attack.  However, this increase in alpha will over-shoot the new equilibrium trim point and the short-period oscillation will occur.  This will damp out quickly (on the order of a few seconds).
At that point, the aircraft will be flying at a new alpha and CL, but will not yet be at the equilibrium velocity for that CL.  The aircraft will be in a slight climb, speed will quickly bleed off while it gains potential energy.   This will over-shoot the new equilibrium velocity and the phugoid oscillation will occur.  The aircraft will trade kinetic for potential energy until drag damps out the excess energy and a new equilibrium velocity corresponding to the lift coefficient is achieved.  The phugoid will damp out over the next 30sec to several minutes.  Aircraft with excellent L/D will take longer for the phugoid to damp.
Since nose-up elevator was input, this new CL will be higher than CL0, and the equilibrium airspeed will be lower.  The drag coefficient will be lower, but whether the drag (or power) is less depends on whether the initial equilibrium point was above or below the point of minimum drag/power i.e. were you on the front or back side of the power curve?
Here, we will approximate propeller powered aircraft as having approximately constant propulsive power with velocity -- and we will work in terms of the power curve.  We would approximate jet powered aircraft as having approximately constant thrust with velocity -- and we would work in terms of the drag curve.
If you are on the front side of the curve, your increase in CL will drop Velocity and will increase CD but will lead to a reduction in drag/power required.  At constant throttle, the aircraft will start to climb (as thrust is now greater than drag).
However, if you are on the back side of the curve, your increase in CL will drop velocity and will increase CD -- but drag/power required will increase.  At constant throttle, the aircraft will start to descend (as thrust is now less than drag).
If, instead of holding the new elevator input constant, you were to simply pulse the elevator and let it return to its neutral position (assuming you were flying stick-free trimmed at the start).
The initial reaction would be similar -- the short-period would be excited, but would quickly damp out -- this would excite the phugoid, which would take longer to damp.  At the end of the phugoid, you would return to essentially your initial velocity in steady level flight.  Your altitude might be slightly lower or higher than your initial altitude.  You would not be in a significant climb or descent -- you should return to your initial equilibrium condition.

The increase in angle of attack increases the lift and drag. The airplane will begin to climb. If the elevator control is small enough, and the pilot continues to hold it in without adjusting, the airplane will begin to oscillate around a new equilibrium in airspeed and climb rate. That is to say, the climb rate will increase, which slows the airspeed. The slower airspeed reduces the climb rate, which allows the airspeed to increase again, and the cycle starts again.
This repetitive cycle of cyclically varying pitch and airspeed is called "phugoid oscillations." Airplanes are intentionally designed to dampen phugoid oscillations, so the airplane will eventually reach a new equilibrium, corresponding to the traditional "climb" free-body diagram you are familiar with (or descent, depending on airspeed). How quickly it reaches the new equilibrium will depend on the plane.
Of course, if the elevator control is too large, the airspeed will continue to decay and the airplane will eventually stall.

This assumes the pilot holds the elevator input, right? If he makes the input & then releases it, everything should damp out at about the same equilibrium conditions (maybe slightly higher altitude) as before, yes?

@RalphJ Yes, my assumption here is that the pilot holds the input.

No weight (mass) is added. Assuming the elevator deflection is constant, this is what will happen.

AOA will increase, causing an increase in Lift and Drag.
The increase in Lift, (greater than the weight), will cause the aircraft pitch flight path to increase, (it will begin to climb).
As pitch attitude continues to increase, (Climbing at increasing climb angle), the airspeed will decrease.
As airspeed decreases, the Lift will decrease.
When the aircraft, (climbing), has slowed sufficiently to the point where the Lift at its reduced airspeed is again equivilent to the weight, the aircraft climb angle will stop increasing.
The aircraft, still climbing, will slow even more, and lift will decrease even more, below the weight.
Lift now less than weight the climb angle will begin to decrease.
The airspeed will continue to decrease as the nose, (still climbing), pitches down.
When the descending nose reaches that pitch attitude necessary to maintain level flight, the airspeed will be below that necessary to generate 1 G of Lift, and the nose will continue to fall below level flight.
Nose below the horizon, the aircraft will descend, and airspeed will begin to increase.

Cycle will repeat, with smaller and smaller oscillations, until it stabilizes at a slower than original airspeed, (since you did not increase power, and the higher AOA will have increased induced drag). If you started this at an airspeed above L/D max, you will probably be in a slight climb. If you were below L/D max, you will probably be in a slight descent.


text:
Need help identifying a radial aircraft engine
I work at the Naval Air Station Wildwood Aviation Museum in Rio Grande NJ.  We just acquired what we believe to be a radial aircraft engine that was found by fisherman in the Atlantic Ocean.  We'd like to display it, but need to identify it first.  Any help would be great. It is an 18 cylinder engine.  We have R-2800 double wasp engine 18 cylinder engine on display that is much larger then the engine we are trying to identify. Thank you.

The Bristol Taurus, Pratt & Whitney R-1830, R-2000, and Wright R-2600 are twin-row 14-cylinder engines.
Could you estimate the overall diameter of the engine?  You can readily measure the bore -- that would be helpful.
The bore of the Taurus is 5", R-1830 is 5.5", the R-2000 is 5.75", the R-2600 is 6.125"
18-cylinder radial engines include:
Pratt & Whitney R-1535.  5.1875" Bore, 44" diameter
Pratt & Whitney R-2800.  5.75" Bore, 53" diameter
Wright R-3350. 6.125" Bore, 55.7" diameter

Thanks for the quick response  The cylinder diameter is approx. 5.75"  I counted the cylinders again and there are 18.

Taurus is out, being a sleeve valve. Sounds like R2800 is it. You could probably confirm it by finding an image from an R2800 parts or repair manual that shows the gear train on the accessory cover.


text:
How are canards designed to stall before the main wing?
I understand that aircraft with canards are designed so that they pitch down when at a high angle of attack. The canard is designed to stall before the main wing, thus the canard produces no lift force, and pitches down around the CG, preventing the main wing from reaching its stall angle.
How are the canards designed to stall first, is it aspect ratio, sweep angle, cross section? The Long-EZ has a straight canard and a tapered, swept main wing yet some jet canards are as swept as the main wing.
I’m looking at applying this idea to a RC model aircraft to give it better performance. Just an amateur rc hobbyist.

You forgot “incidence”

How are the canards designed to stall first?

Canards are normally built with a very high aspect ratio: this gives a quite sharp stall and at a quite low AoA. But that's not the whole story. Canard design is a sensitive (and also quite interesting) tradeoff among contrasting characteristics.

Why is canard configuration so sensitive? Let's suppose that, for whatever reason, the nose of the aircraft goes up. In order for the aircraft to be stable, it must respond pushing the nose back down.
When the nose goes up, the AoA of both the canard and the wing increase and their lifts increase accordingly. A bigger lift generates a bigger downwash. Now, the downwash of the wing doesn't bother us that much since behind the wing there's basically nothing important in a canard configuration. But the downwash of the canard does matter to us because it impinges on the wing thus reducing the local AoA of the wing.
So, a nose up movement of the aircraft translates in an increment of the canard's lift plus an increment of the wing's lift although smaller. But bigger lift in front means nose up tendency. Therefore, if the nose of the aircraft goes up, a canard configuration pushes the nose even more up: the canard configuration is fundamentally unstable.
How can this instability be cured? There are basically 3 things which can be done and each one has pros and cons.

The CG is shifted as far as possible from the wing's aerodynamic center (i.e. as forward as possible); this can be achieved for example sweeping the wing back: its weight and its aerodynamic center shift backward and this has the same effect as shifting the total CG forward toward the canard. But! a more forward CG also implies that a big part of the weight must be now lifted by the canard which therefore has to generate more lift and more downwash, exacerbating at the end the just explained instability.

The canard is built in such a way as to generate always less lift than the wing, also taking into account that its downwash blows on the wing reducing the wing's AoA and lift. This can be achieved for example if: a) the canard has a smaller surface than the wing (1:4 for example for the Rutan VariEze); this normally also implies that the canard's span is smaller than the wing's one and its downwash now impinges on a smaller portion of the wing; b) the canard is built with a very high aspect ratio; this gives a well defined stall and at a lower AoA; c) an airfoil is chosen possessing a quite flat $C_L \text{vs } \alpha$ curve around the stall:

This plot from this NASA report shows the lift coefficient of the canard of the Rutan VariEze (circles) in respect to a standard NACA 0012 airfoil (squares): as visible, the stall region is well defined for the VariEze while for the NACA 0012 there's a region after the pick where the lift goes down and then up again; furthermore "canard stall occurred at about $\alpha$ = 13°, whereas wing stall occurred at about 21°".

We take to the extreme the previous solution and we simply download completely the canard; it now generates lift only to control the aircraft and only in a relative small amount and its shape can be now more or less freely chosen; the instability problem due to the downwash is gone but now there's another problem: if the canard does not lift anymore, then the whole weight is supported by the wing alone i.e. the CG is now very close to the aerodynamic center of the wing; unless the wing is designed self-stable, this condition is not only unstable (again) but this instability is normally quite fast as well and the only way to counteract it is via an automatic stability system which is not really an option on small general aviation airplanes.

If you see canard aircraft with a relative big canard surface, think about the point 1.
If you see a Rutan's aircraft, think about the point 2.
And if you see an Eurofighter, think about point 3.

Are you sure you read the plot correctly? Could it be that the canard lift is referenced to the main wing area, making your comparison misleading?

@PeterKämpf Yep, you're 100% right: they call $C'_{L_c}$ the lift coefficient in respect to the canard surface, I totally missed the single quote. I'll update immediately the answer.

@PeterKämpf: done, thanks for spotting it

I'm still unhappy with your answer because it is plain wrong. The canard lift coefficient is higher than that of the wing. This makes it stall first, so the plane pitches down.

@PeterKämpf: done! I posted the cl Vs alpha of the canard, this time the right one

An airfoil stalls when its angle of attack (the angle at which the airfoil meets the incoming wind) exceeds its critical angle of attack. So, it's simply a matter of arranging it so the canard reaches its critical AoA before the main wing.
One simple way to do this is to rotate the canard back by a tiny bit. If the canard's angle of incidence (the angle at which the airfoil is tilted relative to the body of the aircraft) is higher than that of the main wing, then its AoA is going to be higher by the same amount.
Another (probably more common) way is to use a different design for the canard that has a lower critical AoA.
Of course, there are more esoteric options available, such as designing the body so that the the airflow at the canard has a slight upward motion, increasing the canard's effective AoA. But that would depend heavily on the exact shape of the body, and wouldn't be possible for most designs.
Fun fact: On conventional tailed airplanes, the center of gravity is in front of the center of lift of the wing, meaning the tail actually has to generate a slight downforce to keep the plane balanced. The designers of those planes use these same techniques to ensure the main wing (at least partially) stalls before the tail, for exactly the same reason.

The airflow at the canard has a always an upward component due to the presence of the canard itself (at least at subsonic speed).   On aircrafts of conventional design, tail stalls definitely after the wing for understandable safety reasons: having a wing producing a not-controllable aerodynamic force is not funny

@sophit Ah, yes, I got the stall part backward. But do you have a reference for "he airflow at the canard has a always an upward component"? How can the airflow already have an upward component when the canard is the first thing it reaches?

I simply mean the upwash :)

How are canards designed to stall before the main wing?

By making the airplane longitudinally stable. When canard and wing stall at comparable angles of attack, the canard will stall first when the whole airplane is longitudinally stable. Stability is achieved by flying the canard at a higher lift coefficient than the wing, which in turn is made possible by a larger angle of incidence (calculated from the zero-lift angle) of the canard.
The same is done on conventional airplanes by giving the wing a higher incidence than the tail. In both cases the rear surface has a smaller angle of attack and flies in the downwash of the forward surface, reducing the local angle of attack again. A downforce is not required. In case of a canard the downwash only affects the inner wing and the outer wing flies in the upwash of the canard wake, making a canard less robust against a roll departure near stall.


text:
When does true altitude equal indicated altitude?
Under what conditions is true altitude is equal to indicated altitude? When the sea level pressure is 29.92 in Hg and the temperature is standard for a given altitude, for example 9 degrees at 3000 ft? Under what other condition(s) they are both equal? Or is that the only one?

The first part is one of the many poorly-worded test prep questions in the pilot training world. Yes, standard pressure and standard temperature are the only two conditions they are looking for.
Pressure and temperature are the only significant variables. Humidity has some measurable effect on density but it is relatively small and difficult to predict so ignored in practical flight. Specifically, the lapse rates are important, but the lapse rates can only be approximated for practical aviation purposes.
It is possible for real-world temperature and pressure to diverge away from standard atmosphere such that they will have counteracting effects and result in a true altitude that matches indicated altitude at one specific point, but that is not generally useful to a pilot in flight. However, it is really similar to the source of the altimeter setting for an airport. The pressure setting is(for higher elevation airports) often not the setting that would be found at true sea level, it is the setting that yields an equal indicated altitude and true altitude at the airport elevation(but usually no other altitude). Again this is due to non-standard lapse rates.

Your answer brings up a point which seems to be less well known for some reason, which is that accurate altitude in relation to the ground is more important when you are somewhat near the ground, but at higher altitudes what is more important is accurate altitude in relation to other aircraft. Which is why above 18,000 feet everyone sets their altimeter to 29.92. So if one plane has an indicated altitude of say 29,000 feet and another aircraft has an indicated altitude of 30,000 feet they can be relatively sure that they have 1,000 ft vertical separation. Not that the OP is unaware of this.


text:
When does true airspeed equal indicated airspeed?
When would the TAS=IAS/CAS? When at sea level, pressure 29.92, and 15 degrees of temperature? What other realistic scenario both of these airspeeds are equal? Because it would be extremely rare to at any given altitude have a pressure of 29.92 and have a standard temp for that altitude I guess right?
Thank you!

True airspeed equals calibrated airspeed whenever the density altitude is zero. This occurs at sea level on a standard day, but can also occur at higher altitudes if it's cooler, or lower altitudes if it's warmer. For example, the density altitude is about 0 at a pressure altitude of 1000 feet when the temperature is $5^\circ\rm C$.
The relationship between indicated airspeed and calibrated airspeed depends on the aircraft. Commonly CAS and IAS are quite close at cruise but quite different in slow flight.


text:
How much did Avgas cost in 1940?
I am interested in historic fuel prices;
How much did Avgas cost in 1940?

What country and what area you are asking for? Fuel prices can vary greatly within a single city. You can look here to see prices for last 30 years in the US.

Possible Duplicate: Where I can get historical jet fuels prices?

@Farhan - I thought it might be a duplicate too after reading the title, but the question asks for a specific number, pertaining to AvGas rather than kerosene, in a specific year (and one that goes much farther back than what's available thru EIA, unfortunately). If anything, it might be off topic here and a better fit for SE.History.

Was it even called Avgas in 1940? 100/130 is essentially an ultra-high-octane version of pre-1975 leaded gasoline, and as of 1940 it was only used in highly-supercharged military engines; most civvie aircraft of the day ran on more or less the same stuff that went into cars.

Ultimately I am curious about the price in Guatemala but I figured that would be very difficult to obtain so I thought of making an estimate based on a price in, say, Texas. I have found on the internet that automotive gas was about 11 cents / gallon but I'm not sure if the higher octane version used for aircraft would be similar. Thoughts?

This is what [history.se] would call a counter-factual question. AVGAS did not exist in 1940, which makes the question non-answerable. OP, please edit it. (You might ask "How must would AVGAS have cost..." or "How much was high octane...")

During the war, fuels of all types were heavily controlled materials that could not be bought on the open market, so there was no direct prices that would be meaningful in modern terms.

100 octane fuel was the best possible aviation fuel at that time,introduced in April 1940 .By December 1942, production of 100 octane gasoline had risen to 5.5 million gallons/day; by June 1943, it was approaching 10 million gallons/day; and, by VE-Day, May 7,1945, production exceeded 20 million gallons/day. Avgas was introduced near 1970s. And, then it is very difficult to find exact price for that . But, the at that time the price of car fuel and aviation fuel were not very different. So, you can estimate it to be at 10-12 cents/gallon range. One other way to estimate the cost is by using the inflation adjusters, the current price for 100LL is about $5.5 . So, by using this calculator, it would have cost nearly 10 cents/gallon. (In 1940, Dollar and Pound were nearly same in value.) So, the final answer should be about 10 cents/gallon.

What locale does this answer apply to?

That's northern US, where average price of 100LL is $5.5 ..

Do you really mean "0.10 cents/gallon"?  That's one tenth of a penny per gallon, which seems a little low, even for the "good old days."  Or do you mean 10 cents = $0.10 per gallon?

Not really..!! It was a silly typographical mistake..

Terrific- thank you for your responses!

Welcome! But we are more concerned about your response!

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Flight plan routing - when can you go direct?
When you are creating a flight plan and getting a route from airport A to airport B, I noticed that in flight simulation the routes will mostly have a pattern of going to a waypoint --> then an airway --> to an exit waypoint --> to the next airway --> to the exit waypoint.  E.g. KONAN UL607 KOK M150 GEBDA Y173 PABLA
However, I also noticed that occasionally the flight plans will start routing direct from one waypoint to another (i.e. making up their own airways), e.g. BIRGI DCT INGID DCT GOTAR DCT NALOX DCT PARAK
Now what I don't understand is whether in the real world that is even allowed? If it is allowed, then in what circumstances can you do that and are their any rules and restrictions about doing it? E.g. if I want to route direct to another waypoint, then it must be within 10 NM of your current waypoint.

It would help if you included the jurisdiction (EuroControl, NavCanada, etc) so that answers can be specific to that system.

Well that's a good point, because I am programming routing algorithms that I want to work worldwide, I didn't even think there would be different rules in different regions? I'm trying to come up with an algorithm that can generate accurate routes across the world.

Thanks, that is helpful to understand the Austria situation, I guess the answer is that it's fine to direct route, but it depends on where you are?

@KillerKode It will probably depend on where you are. I can't speak for anything other than USA flight plan processing, but I doubt the automation is consistent across jurisdictions.

The "also..." portion needs to be deleted and asked as a separate question. All SE sites work on a one-question-per-post basis...

Forgive me, the "also" is not an additional question but poorly presented here so it seems so.  I added the "also" part because I thought it may be a clue / related to the answer of my question.


text:
Why do helicopters have skids?
Why can't they just land on the helicopter's floor? If it's for stability, they could still use skids which are shorter than what is commonly used.

Also the skids provide clearance, walking into a spinning helicopter blade will kill you in a particularly graphic manner.

While I'm not a helicopter designer or mechanic, the skin of aircraft tends to be rather thin.  Get a wind gust when you're landing your skidless chopper, so you scrap the belly along the pavement, and you're faced with an expensive repair.  Scrap the skids, and worst case you just unbolt it and install a new one.

What else is James Bond going to hang on to?

@jamesqf Even if it weren't thin, let's say the body of a helicopter were similar to that of a car - landing on its belly would definitely ruin the paint job.

Just imagine how goofy you would look like, when trying to get in and out of that, with the huge risk of hitting your head on the fast-rotating blades.

Although amphibious helps do land on their belly during water operations.

Image source
Larger helicopters have landing wheels, and as we can see on the picture above, they are placed as far outward as practically possible in order to provide stability in the landing. Touch down one wheel on an sideways inclined slope, lower the collective, and the helicopter will align itself onto the slope with:

The location of the Centre of Gravity (CoG) always in between the wheels. The higher the CoG, the wider the landing gear impact points must be.
The weight being supported on clearly defined and structurally reinforced points. Even if the bottom of the helicopter is perfectly flat, the landing ground may not be, and to support all of the weight at any random point at the bottom requires a lot of relatively useless weight for the re-inforcement.
The behaviour after first wheel touchdown being clearly defined. The wheel is a point of impact upon the landing plane, mathematically much more defined than a plane of impact upon a landing plane if the latter is curved.

All the above is for wheels, which allow the helicopter to taxi on-ground. Skids perform all of the above functions apart from the on-ground taxiing, and are cheaper and lighter than retractable landing wheels. Of course, the other downside is that they have more drag.

One would not want to land this helicopter without the skids!

Retractable wheels on helicopters do exist though. So if you don't see skids or wheels, the helicopter probably has those (like this SAR Eurocopter Dauphin from the Dutch coastguard).

Yes they do indeed exist. Short & stubby & easy to retract.

@Mast Airwolf immediately popped in my head when I read that.

Physically they likely could, but helicopters (and all aircraft for that matter) tend to have stuff mounted to the bottom like antennas, probes, cameras etc. so adding some kind of landing gear or skid helps to provide clearance for all that. This question covers a bit about whats mounted where.
The belly of a helicopter also tends to not be perfectly flat (depending on the model) so belly landing can lead to some roll which may allow the rotors to scrape the ground.

Skids also have a bit of spring to them to provide a little give when landing.

Skids are especially beneficial when the helicopter is supposed to land on rough ground, e.g. grass.
They move the body and the rotors farther away from the ground, thus giving both more clearance.
Compared to extending the body to the ground to achive the same effect, skids add less weight and significantly less air resistance.
Compared to wheels they are better suited for rough ground, as they have a larger contact area and thus tend to sink less into the ground.
Wheels on the other hand have the advantage when the helicopter usually lands on concrete or other solid ground. Here they allow taxiing.
That's why you tend to see skids on smaller helicopters which tend to land on grass or other uneven terrain, while larger helicopters, which tend to land on more solid ground, tend to have wheels.
Also, skids are simpler than wheels, which also suits smaller, cheaper helicopters better.

Skids, as opposed to wheels, allows the pilot to get some sense of balancing tail rotor counter torque before lift off.  Skids would provide more drag until the pilot "got it". Same thing moving forward, backwards, or side to side.  The skid friction limits movement (or damps it if you will), rather than creating a nightmare of correct/overcorrect in 2 dimensions.
Skids also add an important amount of rotor clearance, and offer simplicity, reliability, and light weight.

Why would wheels not allow the pilot to get a sense of the torque provided by the tail rotor? Brakes are a thing, and the heavier helicopters (which have higher torques to counter) all tend to have wheels, not skids.

@AEhere supports Monica wheels would provide a sense of torque, but could quickly become unmanageable with a ham handed novice.  Larger helicopters have more inertia, there for would react slower.  Wheels with brakes are more complex.

An additional benefit of longer skids is some energy absorption upon a vertical crash. The skids are generally designed to bend outwards and up during impact. This can be seen occurring in many helicopter crash videos. The design of impact absorbing skids is covered extensively here.

not only during a crash.. they absorb some of the force of landings. Simpler and cheaper than landing gears.

Also, they presumably allow the copter to... well, skid forward a bit relatively safely, if the pilot can't prevent a bit forward motion during touchdown. If they were e.g. transversal bars or a grid instead, that would dig into the ground, causing the risk to tip over to the front.

As pointed out above, not all helicopters use skid style landing gear.  Some have either a fixed or retractable wheeled undercarriage.  These offer excellent mobility on paved or short cut grass airfields, surface taxiing, rolling takeoffs or landings or for movement aboard ship.  Surface taxi also consumes a little less fuel than hover taxi or takeoff from an IGE hover, but are more complicated, heavier, and expensive than skid type landing gear.
Smaller, utility helicopters usually use a skid type landing gear For several reasons.

Lightweight.
Inexpensive.
Flexible and can absorb impact of a hard landing or crumple to absorb energy.
Easy to replace if damaged.
Provides a large, stable support for pinnacle or slope landings.
More even distribution of aircraft load for landings in soft or waterlogged fields.
Lends itself better to being fitted with fixed or emergency inflatable floats for over water operations.
Can provide support for personnel flying outside the airframe.

The downsides to skids are

Do not allow the helicopter to be easily moved or towed on the ground; must be fitted with towing dollies or placed on towing trailer.  Surface movement limited to hover or air taxi.
Are fixed and cannot be retracted and faired, thus adding more drag in flight.
Easier to snag on obstacles and wires during surface operations.
More susceptible to side loads during takeoff and landing, which can cause dynamic rollovers.  Great skill must be used during rolling takeoffs, shallow descent landings or autorotation to keep the aircraft moving in a straight line down the runway to prevent side loads.

The aircraft’s fuselage is shaped primarily to provide the best aerodynamic fairing for the cabin, engine and transmission and aircraft systems; it is not necessarily the most stable platform and does not provide any shock absorption in case of a hard landing or an autorotation landing.  Landing gear is designed to solve that problem.
The siding of the skids is based on a lot of things from the ability to provide solid, stable support on uneven terrain to prevent the onset of potentially deadly ground resonance during a harder than normal landing to offering good ground clearance.


text:
Would "Ground Control" be feasible for commercial aviation?
Would the airline pilots be able to get instant technical assistance from the ground?
Today, ground systems receive only a tiny trickle of information while the airplane is in flight. But with communication channels steadily improving (systems like 2Ku promise satellite bandwidth in 10s MBps range), we should be able to channel enough data to almost make the flight data recorders redundant.
Can we go one step further and set up a NASA-style mission control centers for commercial flights? The workflow should go approximately like this:

The plane is flying and constantly transmitting flight data to ground control center at low bandwidth (but still higher than is practiced today);
When plane's systems detect any kind of abnormal situation, data feed is switched to high-bandwidth, and ground control center is alerted;
Software at the ground control center processes the alert as well as expanded data stream and may decide to escalate the situation to human personnel;
Human personnel reviews the alert and may initiate a conversation with plane's pilots. Pilots can request assistance from ground control at any time;
Ground control specialists can request the plane to stream specific data, depending on situation;
In the end, instead of two pilots, whose training may be questionable, we have a team of specialists working to resolve the situation.

Today, air traffic control can help the pilots somewhat, but they don't have any data feed and can't help with technical issues. Airline companies
collect some flight data, but only a little. I don't know if it's a norm in the industry to maintain 24/7 "ground control centers". What is clearly different in my proposal is that pilots don't have to explain everything to ground control. Ground control would have all the data and should be able to advise pilots sometimes even before they are aware of any emergency.
Do you think the plan above would be helpful and technically feasible?
P.S. My question is different from "Could the CVR and FDR record to the cloud?" because I'm not looking for 100% data dump (transmitted data would always fit the available bandwidth) and actually look for a more advanced solution of taking a real time action by the ground control.

Technically feasible, of course.  With enough money and a commitment from the industry almost anything is possible.  And while it sounds like a good idea at first, I am struggling to envision a scenario where the benefits of such a system would exceed what can be accomplished with a trained crew assisted by ATC under the current system.  Can you offer a specific example of a situation where streaming data to the ground might help the pilot?

A lot of this is already in place. For example, Collins Aerospace has ARINCDirect which is installed on a majority of commercial flights that includes the ability to monitor aircraft health and fault reports. Many pilots can contact "company" directly during a flight, either by phone, CPLD, VHF or other means to help diagnose issues in-flight.

"24/7 ground control centers" broadly exist in the form of airline dispatch offices, which track and monitor flights and coordinate communications with the crew when there are issues, though not with entirely the full stream of real-time telemetry you contemplate (there is also engine telemetry, which may be sent to the manufacturer to determine future maintenance needs). So I'd read up on what dispatchers do. But it's hard to contemplate how this is helpful in an emergency where immediate action must be taken, and if there's no urgency, then things can be talked through as they are now.

Possible duplicate of Could the CVR and FDR record to the cloud?

@Michael Hall I can think of a number of accidents that could have been prevented. For example, both 737 MAX Boeing crashes (Lion Air Flight 610 and Ethiopian Airlines Flight 302) as well as Helios Airways Flight 522. However, if the accident was preventable, we can always argue that it was a pilot training issue.

@Alexander, ok admittedly I haven't read the full NTSB report, but to oversimplify you think that by streaming data to the ground a team of experts could have recognized what was wrong and told them to turn off the autopilot and pull out of the dive, thus preventing tragedy?  I'm just not seeing how people on the ground could react quickly enough to help in a rapidly deteriorating situation.

@Michael Hall in case of 737 MAX, the situation had been unfolding during a few minutes, and in other instances more experienced pilots had been able to handle it better (though not to pinpoint the problem). I presume that a Boeing expert, upon seeing the flight data, would have immediately suggested to disengage MCAS. In an emergency, ATC today is trying to do a similar job, but they are blind without the data.

@Alexander I think you misunderstand the purpose of ATC; it is not their job to tell the pilots how to solve their issues, simply because they cannot know the details of every aircraft type in existence well enough to provide input. In an emergency, ATC is there to help the aircraft navigate, deconflict the airspace and relay communications when needed, not to tell the pilots to disengage the autopilot. I would also question your optimism about the characteristic time of such an "expert assistance system".

@AEhere I don't mean it's ATC's job to provide technical assistance, I only mean that in case of emergency ATC is trying to do everything to help the pilots. As far as "characteristic time" of expert response goes - this is what I'm trying to find in my question.


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Can a non-pilot realistically land a commercial airliner?
It's a situation that often played out in the movies - the pilots are incapacitated, and the person in the cockpit is looking at all of those instruments and controls, wondering what to do with them.
How much of a chance would this person have?
Assume that the person can successfully communicate with Air Traffic Control over the radio.
This is closely related to Can a passenger realistically replace suddenly incapacitated pilots?, but I want to stress that the person in cockpit would have no prior flying experience whatsoever, while the plane has well-functioning autopilot and autoland systems. Can a modern plane be landed without ever touching the throttle and control stick?
P.S. A clarification on why I assume that communication with ground should be realistic. Many commercial planes today have WiFi on board, and people, even if not voice calling, are able to actively use their social media accounts. A video from the cockpit with appropriate commentary will instantly attract many views (and, however perversely, "likes"), so the person who sent it would eventually get in contact with appropriate authorities and get necessary instructions.

Most people will have no idea what frequency to use to talk to the ATC, since it changes by area. Assuming they have established contact, the person in the cockpit will have to be guided on every step, and execute the instructions precisely.
The term is "talkdown landing", which have been done with GA aircraft, but have never been tested with an airliner in real life.
During the Helios Flight 522, a flight attendant, who happened to be a commercial pilot (one step below airline pilot lice3se) attempted it. However, he was on the wrong frequency, out of time, and possibly suffering from hypoxia, all factors playing against him.
The TV show Mythbusters attempted a talkdown airliner landing in a simulator, and were talked down successfully, proving it's at least sometimes possible. There is a video online. Note that the show's presenters are highly technically skilled.
University of North Dakota ran another test recently. The participants had access to a radio for talkdown landing. Out of 5 people, 2 failed and 3 succeeded. The 3 that succeeded included a commercial pilot, a private pilot, and a flight sim enthusiast. The conclusion seems to be that a little bit of training, even on the wrong plane, goes a long way.
There isn't a single "land now" button, except on a few business jets (not airliners). The operator still needs to set correct heading to reach the runway, activate autoland, make sure it homes in on the localizer, extend the landing gear. The throttle is automated, except for reverse thrust, but it's an easy control compared to the switches.
Without external instructions, it's not realistic at all. Attempts to land without communication have failed in experiments above, as well as in real life. Everything's possible - there's even been a case of a plane (not an airliner) landing itself by accident - but this one is highly unlikely.

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Is it theoretically possible? yes.
Is it likely to work? no.
This is assuming all aircraft systems are functioning normally, and for whatever reason both aircrew just dropped dead in their seats during cruise phase with the cockpit door unlocked and open and someone noticed them being dead. It also assumes the runway they're going for has a CAT III ILS system operational (many do not).
If the aircraft is one in the A320 family, until just before touchdown no input to the controls is needed except on the glareshield and MCDU from the moment you engage the autopilot shortly after takeoff. Once the aircraft reaches cruise altitude, "all" the pilots need to do if everything goes well (and it almost always does) is monitor systems, enter the arrival information into the MCDU, set the final approach altitude on the glareshield, and then when prompted press the altitude button to go into managed descent mode.
Then later they need to drop flaps and landing gear as they approach the field, and arm ground spoilers and autobrakes.
Then shortly before touchdown (as prompted) they would have to pull back the throttles, and on touchdown engage thrust reversers. On stopping (let's assume they're not going to attempt taxiing to a gate or hard stand) they'd have to shut down the engines and engage the parking brake.
They COULD be talked through this by a pilot in the tower, IF they figured out the correct frequencies to program in the COM radios (paper charts would help, let's hope the pilots managed to open their chart books to the correct pages before they died).
With most any other aircraft with less automation, it gets progressively harder and less likely to succeed. And if the death of the pilots were caused by something that's wrong with the aircraft, good luck as you now have damaged systems to manage as well as your panicky unqualified people at the controls.

Thank you! That sounds that for A320 at least, getting on the radio would be the most difficult part :)

Mentour Pilot and Tom Scott both have videos showing a non-pilot (Scott) attempting just this in a B737 simulator (similar to the Mythbusters episode), with Mentour Pilot acting as ATC. In the video, they attempt two scenarios - one using the autopilot (and autoland) and one without. As you can imagine, the one not using automation failed, but the autoland did exactly what it was supposed to. The only control-input-type-things the passenger-turned-pilot did was to add flaps, put the gear down, and keep the aircraft centered on the runway after touchdown. Everything else he did was to change settings on the autopilot.
In the videos, they note that there are one or two major caveats with this experiment. A passenger on an airliner is very unlikely to even get into the cockpit in the first place, making the whole question rather moot. And, if someone were to get in (maybe a flight attendant?), figuring out how to communicate with ATC could be problematic.
In the general aviation space where cockpits aren't secured like on airliners, there is the Garmin Autoland system which will, at the press of a button, automatically land and stop the airplane. It's certified in a handful of aircraft at this time. There's also a ballistic recovery parachute, popularized by Cirrus aircraft (where it's standard); this system deploys a big parachute to bring the aircraft to the ground gently (enough that in some cases the aircraft can be repaired and re-flown!), and is activated by pulling a handle.

There are also ballistic chutes.  By my understanding, they are designed to "land" an aircraft well enough that the passenger and crew are likely to survive which--if the pilot is incapacitated--could probably be called a "win".

Thank you! Just a little note - I never specified that the "non-pilot" must be a passenger. In a commercial airliner scenario, that would likely be a flight attendant.

Well with a novice at the controls or not, it will be coming back to terra firma, one way or the other.
Tongue in cheek aside, depending on the A/C type, a person with no flight time or experience flying an airplane with have a tremendous struggle doing this and will probably result in a fatal accident.
With limited flight training and a skilled pilot to help talk you down, most likely a neophyte can get an airplane back to a runway and land it with minimal damage.  This has happened on several occasions with general laviation aircraft, such as this case where a woman was talked down to land a Cessna 421 after her husband had a fatal heart attack at the controls.  It has also happened on at least two occasions with military jet fighters, once in 1959 when a USAF mechanic became airborne in an F-86 at night, then in 1986 when a Marine mechanic took an A-4 up for an unauthorized joyride.  It is reasonable to assume that an inexperienced person could, therefore, land an airliner like an A320, if they could be talked down by an experienced A320 captain and instructor pilot.
I would also argue, as in the cases above, this is only realistically possible under the following conditions.

PIC would have to be a very quick learner and make a lot of perceptions and insights in a very short time.  And all this must be done inside an airplane, and they make terrible classrooms.
Airplane would have to be in working order, no damage or system malfunctions.
Visual Meteorological Conditions would have to prevail throughout the arrival and approach; flying an instrument approach would be an excessively tall order for a newbie.
Winds must be light; crosswinds with really complicate things.

But no matter how much external help they had, most likely the roundout, touchdown and rollout are going to be pretty rough, possibly with the aircraft exiting the runway.
The biggest problem is, even with the best knowledge base and people to talk them down, they still lack the kinesthetic hand-eye coordination needs to successfully fly the airplane to the runway and control it in the rollout.  This coordination or lack thereof would most likely make it very difficult or impossible for a newbie to land a helicopter due to the consequential nature of making control input and the ease of entering very dangerous flight phenomenon during approach like a vortex ring state.

"flying an instrument approach would be an excessively tall order for a newbie" - I presumed that with ILS Cat III, pilot (or "newbie pilot") would only need to engage autoland and respond to prompts (apart from concerns like crosswinds which may force to abort the landing), no?

Very few airports in this country have those approaches available.

Is there a list of airports with Category 3 ILS systems?

In addition, those approaches require specific training and certification to fly plus ground equipment, which can handle those kinds of approaches. It would be a hell of a lot for a neophyte to handle, especially in hard IMC down to minimums.

Is your point that, for a newbie, operating an autoland system would be more difficult than landing an airliner manually?

My point is that there’s a reason that regulatory agencies require a pilot hold specific ratings to fly such an approach. And they are already going to have their hands full of managing the airplane without having to worry about that. You try something like that in hard IMC,  you’re asking for trouble.  You cannot assume that somebody is going to be able to set an autopilot up like that and fly that on the first try.

It's theoretically possible without an autoland system. A passenger climbs in the pilot's seat and broadcasts a call if they can figure out which button is for the mic, which will be on a center frequency, which gets the ball rolling.
Someone intimately familiar with the airliner and its  autopilot system could, if the passenger was smart enough and able to cope with the pressure, instruct the person to push this, button, then that button, on the autopilot panel and the Flight Management System keyboard, move the trust levers to some position, whatever it takes and get the autopilot to navigate to a runway and intercept a final approach glide path.
It can all be done with button pushes and knob twiddles, and working the thrust levers back and forth, right down to a landing flare before they even have to touch the control wheel.
Assuming you don't have autoland, the only physical hand flying challenge is managing the thrust during the approach if there isn't an autothrottle system, and the flare and touchdown.
You have them leave the autopilot to fly the approach right down to 100 ft or so, tell them to hit the A/P disconnect button, and start to pull as the GPWS radar altitude callouts are counting "50, 40, 30...". Pull back the right amount, keep the wings level, and it will land in one piece, maybe hard, but in one piece. Then it's a matter of standing on the brakes and steering the thing like a soapbox derby car.

Given a long and wide runway, would people be more likely to survive an inexperienced person's attempt to flare the landing of a Boeing-style jet, or simply following a minimal-speed glide slope into the ground?  I'd expect landing gear would need inspection if not replacement on a non-flared landing, but an attempt to flare the landing in a non-Airbus-style plane would seem like it would run the risk of gaining too much altitude and then stalling, resulting in a loss of control authority.

A touchdown at 500-700 fpm would cause a bounce and blow all the tires and probably do structural damage, but you'd survive it likely.

The Washington Post recently asked that question and put several novices through a flight simulator (https://www.washingtonpost.com/travel/2023/05/20/passengers-try-landing-plane-flight-simulator/ - there may be a paywal).  As you might expect, the results were not good.
Their summary:

Based on our simulator experiment, no inexperienced traveler should ever volunteer to land a plane in an emergency. Even with a prodigious amount of guidance, which Wilson said was highly improbable in a real-life scenario, our recruits still cratered.

However, if there are no other options, remember these invaluable lessons: Never disengage the autopilot (don’t move the side stick or press the red button). Put on the headset, and hold the switch when you speak. And take five seconds to sip your proverbial tea.


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Why don't we use hydrophobic (icephobic) coatings over traditional anti-ice methods?
I am a design engineer on GA aircraft and I had a thought whilst flying in icing conditions the other day. I know there are lots of ways we deal with ice buildup (as per the chart below from this post, many of which are complex in nature and require more attention in design and operation as compared to passive coatings.
Why don't we use hydrophobic (or rather icephobic) coatings instead of using the more common methods like deice boots, TKS or bleed air? I would think a strip of hydrophobic material such as this 3M™ Microfluidic Diagnostic Film or a hydrophobic paint additive that covered the leading edge would work on most general aviation aircraft. I am sure there are downsides that I am not seeing otherwise it would be used much more.

A successful hydrophobic material needs to be able to withstand temperatures from -50° (at altitude) to 110° (on a desert ramp in the summer), and be durable against light debris (dust and insects) impacting at over 100 mph, while exposed to corrosive pollution (fuel and oil spills, even just tiny drops).  Have you considered these effects?

@abelenky Good point, but yes I have thought about this. We use a material called speedtape a lot on aircraft and it you just vary the thickness and hardness of the tape to which part of the aircraft it is applied to. A sufficiently thick (~.030-.050") should suffice for the abuse it will take not to mention that you can replace it at given intervals.

@abelenky: And it has to last for 50 years or more.  (Or be readily replaceable.)  Most plastics tend to degrade from prolonged exposure to sunlight.

-50 degrees celsius will not be enough. Just flew over Greenland, it was -66 deg C at 11000 ft.

@JoeBo the product you provided a link to is hydrophilic, not -phobic.

I have no doubt that a lot of experimentation has been done using coatings on GA airplanes and to the extent that a coating would work, the main problem that I see is degradation of the coating from UV and erosion.  A coating would likely have to be restored regularly and would be likely too expensive for most tightwad GA pilots.
The other thing is that clear ice that forms quickly can tend to sit in place because it's "cupped" around the leading edge and a non-stick surface may not help since the ice shape can be more or less mechanically held in place even if it's unable to actually stick to the surface.  Rime ice that tends to build up out in front of the nose of the LE would probably shed from a non-stick coating a lot better.
On a side note, your chart doesn't include "evaporative" anti-ice, which is a sub-type of heated anti-ice.  On jets this is often used with hard (non-slatted) leading edges because of the need to avoid "runback ice".  A leading edge hot enough to melt ice, but below the boiling point, will allow the water to flow back and re-freeze farther back on the non-anti-iced part of the wing.  Anti-ice systems in that case will be evaporative, that is, so hot that water immediately vapourizes when it touches (like a steam iron).
Such anti-ice systems run at 105+ C temperatures.  A coating that could prevent adhesion of runback ice behind the LE might be quite useful and could allow those wings to run lower temperature systems (the heat plays havoc with the LE structure over the long run).
Runback ice is tolerable on slatted wings and those anti-ice systems will run a lot cooler, around 50 deg C.
On yet another tangent, I always used to wonder why some jets anti-iced horizontal tails and some didn't. Turns out you don't have to anti-ice the surface if you make it big enough.

And for many GA pilots, encountering icing conditions is a rare thing.  In maybe 40 years of flying, I've encountered ice exactly once.  (During instrument training, with my instructor along.)

Problem for GA aircraft without cert for fight into known icing is, if you want to travel around IFR, you are pretty much stuck below the freezing level, which means in winter you can only fly in VFR weather anyway, or maybe very thin cloud layers you can get through quickly.

the thinnest of oily dirt films atop any such anti-wet coating will render it totally ineffective. There is no way in practice of maintaining that coating in an atomistically clean state.


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Does the aerodynamic center always have to lie on the chord line?
A lot of resources online state that the aerodynamic center is the "point on the body of an airfoil with respect to which the moment coefficient does not change with angle of attack." However, does this necessarily imply that the aerodynamic center must lie on the chord line? Of course, we can resort to thin airfoil theory and prove that it lies on the chord line, but what about for highly cambered airfoils?
This answer (Why does the aerodynamic center exist?) explains the existence of the AC, but resorts to inviscid, incompressible, thin airfoil assumptions. What about in general though? So, is the aerodynamic center present on the chord line in general for all flows? If not, when does it deviate from chord line and why?

I don't know enough about the mechanics of aerodynamics to provide a thorough answer, but isn't the aerodynamic center given as a % of MAC? Therefore it wouldn't be on the chord-line unless it was 100% of MAC.

Yeah, I don't understand how the aerodynamic center could be anywhere but along the chord line.  How could it be somewhere else?

No, there is no reason why aerodynamic center (AC) have to lie on the chord line. As the OP correctly pointed out, Thin Airfoil Theory (TAT) result indicates that the AC:

Exists, and
Lies on the chord line 1/4c away from the leading edge.

However, this is only because the TAT makes the fundamental assumptions that the airfoil is thin, the mean deviation of the camberline is small relative to the chord, and small free-stream incidence (AOA). As shown in the diagram below, the vortex sheet is assumed to lie on the chord line instead of the mean camber line. (I should mention, however, that the slope of the mean camber line is accounted for in the theory to construct the tangent boundary condition on the airfoil). And, of course, TAT doesn't account for drag.

(Diagram cited from Anderson, Fundamentals of Aerodynamics.)
The vertical offset is small, however, at least until the drag rise associated with flow separation or when flow incidence becomes large. If there is a large vertical separation between the center of pressure and the chord, then AC will not exist and this manifests as nonlinearity in the Cm curve.
I've included the pitching moment coefficient about 1/4 chord of NACA0010, Clark Y and NLF(1)-0115 analyzed with Airfoil Tools, which were analyzed with xfoil using panel method + boundary layer equations. The plots included Reynolds number of 500,000 and 1,000,000 with Ncrit transition of 5 and 9. Note that AC exists locally where Cm has a constant slope.

For NACA 0010, AC exists up to 10 deg AOA at 1/4c; near stall, the drag rise occurs and the Cm becomes nonlinear. For Clark Y and the NLF, the AC is pretty close to 1/4 for small AOA, then shifts to a point further aft until drag rise.
Therefore, while the concept of AC is useful for theoretical derivation of stability relationships in linear aerodynamics, it's not a good enough for precise modeling. In engineering modeling, we generally measure the pitching moment (from wind tunnel) at 1/4c, and account it as a function of AOA.

A legend for the graphs would help for completenes. But still, I struggle to see how could the conclusion ("AC exists up to 10 deg") be derived from these graphs.

@Zeus Right above the graph, "Note that AC exists locally where Cm has a constant slope."

Yes, but what about the bends at, say, 3-5° for NACA? I think they need an explanation, otherwise it's rather confusing...

@Zeus I think you're referring to the NLF? I think it's the end of the laminar bucket. But I can't be sure why it's doing it. I mentioned above that the AC shifts aft past small AOA.

My point is simply that most people looking at these graphs will not find anything special at 10°, but will find many other 'interesting places'...